Disposable containers such as plates, bowls, platters and the like are usually made of plastic, or are pulp molded, or are pressware made from flat paperboard blanks. Most pressware paperboard plates, trays and bowls have a flat, planar bottom area. Some of these products have a downward concave bottom area as a result of paperboard fiber springback after forming. This can result in a “rocker bottom”, or a product that tends to rock on its bottom during use. Some pressware paperboard products have been designed with what is commonly called a “gravy ring” around the periphery of the plate, so as to allow any liquids or grease to accumulate in an annular ring area disposed between the plate sidewall and raised planar central portion. Such designs may ameliorate the rocking problem, but appear to provide only limited additional strength as is seen in the finite element analysis results discussed hereinafter. Note also paragraph 93 on page 10 of United States Patent Publication No.: US 2006/0208054 to Littlejohn et al. (U.S. patent application Ser. No. 10/963,686) which states that while the bottom of pressware containers are generally planar, a step contour or a crown of a few degrees or so may be provided to address the problem of rocking. Pulp molded plates or plastic plates may be formed, as sometimes observed, with a convex (upward) crowned bottom, although it is not clear whether this is an intentional feature or a result of shrinkage after molding or thermoforming.
Pulp molded containers exhibit generally excellent dry strength as compared with many pressware containers; however, pulp molded containers are generally inferior to pressed paper products in terms of coating and decorative options because suitable printing and overcoating processes for pulp molded containers are relatively difficult and expensive as compared with available options for pressware. This is so because paperboard can be coated and printed prior to forming into shape. Pulp molded products are accordingly usually uncoated and not as resistant to grease and moisture as are pressware products with suitable latex coatings. Most plastic or foam plates have a limited heat/reheat range, and can soften or melt with hot foods or during microwave use. Thus, pressware containers are preferred in many cases.
Pressware containers have been produced with various flange profiles as is seen in the patent literature. U.S. Pat. No. 5,326,020 to Cheshire et al. discloses a container with a plurality of frusto-conical regions extending outwardly from the bottom of the container, while U.S. Pat. No. 5,088,640 to Littlejohn discloses a rigid four radii rim paper plate. See also U.S. Pat. No. 6,715,630 to Littlejohn et al. which discloses a disposable container having a linear sidewall profile and an arcuate outer flange as well as U.S. Pat. No. 7,048,176 also to Littlejohn et al. which discloses a deep dish disposable container made from a paperboard blank. Processing techniques and equipment are further detailed in United States Patent Publication No.: US 2007/0042072 to Johns et al. The '072 publication details apparatus and equipment suitable for making pressware at high throughput rates.
Pressed paper plates are typically formed from flat blanks. The blanks may be scored around their perimeter to aid in the necessary gathering of the paper during the formation of the product. The folds or pleats created in the final pressware product ideally are pressed and reformed with heat, moisture and pressure to “rebond” the structure and obtain high strength. However, pleats or folds can still be lines of weakness where hinging or opening can occur during plate use resulting from local flexure or tension, thus lowering the product strength and durability. U.S. Pat. No. 4,721,499 to Marx et al. is directed to a method of producing a rigid paperboard container having rebonded paperboard pleats. Dimensions appear in column 5, lines 12 through 43. See also, United States Patent Publication No.: US 2006/0208054 noted above. The products and methods disclosed in the '054 publication exhibit increased rigidity and rim stiffness as compared with other more conventional pressware products. These containers have an outer flange portion extending outwardly with a brim portion sloping downwardly defining a declivity angle with respect to a horizontal generally parallel to the bottom portion and includes an outward turn at the periphery of the container. This geometry has been found particularly suitable for pressed paperboard servingware. Dimensions of the various products appear on page 12, Tables 1 and 2.
Notwithstanding the many improvements already made in connection with pressware products, there is an ever present demand for pressware products with increased rigidity and increased load-bearing capability.